n it lowered the
ship. Then the explosion occurred beneath the waves. If I had waited a
little while I need not have risked the dynamite."

"Well, there was no guarantee that the rock would go back where it came
from," remarked Jack.

"No, we had to act," agreed the professor. "But now let us go aboard."

They rowed back to the _Porpoise_, which they found had sustained no
damage from the queer experience it had been through.

The motors were set in motion and in a little while the craft was again
moving through the water out of the dangerous vicinity of the volcanic
area.

"Who has the two watches to-night?" asked Mr. Henderson after supper had
been served.

"Washington and Andy," answered Jack, who kept track of the matter.

"I guess we'll change it, and have you and Mark take them," went on the
captain. He gave Jack a peculiar look, and made a sign to him not to say
anything. Wondering what was to come Jack went up on deck to watch the
sunset.

CHAPTER XXI

CAUGHT IN A WHIRLPOOL

The boy was joined a little later by the captain, who, coming close to
him whispered:

"I am going to try to discover the ghost to-night. That is why I wanted
you boys on hand to help me. This thing must be stopped if it is a joke,
and, if it isn't--"

"Do you think it is some one playing tricks?" asked Jack.

"I don't know what to think," answered Mr. Henderson. "We will see what
happens to-night."

Mark came on deck a little while, and the three talked of the strange
appearances of the mysterious white object. The boys told of their
experiences, and Jack related more fully his on the night the ship went
up on the big rock upheaval.

"I don't believe in ghosts," said Mr. Henderson, "I'm going to lay this
one," and he smiled grimly.

Night settled down. Jack, who had the first tour under the new
arrangement, had made himself comfortable in the conning tower, and Mr.
Henderson had hidden himself in the companionway. His idea was to thus
guard both openings into the ship and ascertain whether the ghost came
from within or without the craft.

Up to a short time before twelve o'clock nothing out of the ordinary
happened. The only sound was the lapping of the waves on the steel sides
of the _Porpoise_, and now and then a splash as a big fish leaped out of
the water. There was only the slightest breeze.

Jack who, somehow or other, felt much sleepier than usual, caught
himself nodding several times. Once he awoke w